1. Field of the Invention
This invention relates to a process for improving the mechanical properties of metals, and more particularly to a process for stabilizing an aluminum composite having a rapidly solidified metal matrix and reinforcing phases by incorporation of oxides and carbides through mechanical alloying.
2. Description of the Prior Art
An aluminum based composite is generally comprised of two components--an aluminum alloy matrix and a hard reinforcing second phase. The composite typically exhibits at least one characteristic reflective of each component. For example, an aluminum based metal matrix composite should to reflect the ductility and fracture toughness of the aluminum matrix and the elastic modulus and thermal stability of the reinforcing phase.
Aluminum based metal matrix composites containing particulate reinforcements are usually limited to ambient temperature applications because of the large mismatch in higher temperature strength between the aluminum matrix (low strength) and the particle reinforcement (high strength). Another problem with aluminum based metal matrix composites is that the dispersed strengthening phase is not stable at elevated temperatures, and coarsens after excessive thermal exposure, which in turn leads to a degradation of the materials' mechanical properties. Another problem with aluminum based metal matrix composites is the difficulty of producing a bond between the matrix and the reinforcing phase. To produce such a bond, it is often times necessary to vacuum hot press the material at temperatures higher than the incipient melting temperature of the matrix. It has been proposed that this technique be avoided by mechanically alloying the matrix with the addition of particulate reinforcements. This procedure, referred to as solid state bonding, permits the reinforcing phase to be bonded to the matrix without heating the material to a temperature above the solidus of the matrix. Moreover, it has been further proposed that mechanical alloying be performed with the addition of a carbidiferous agent, e.g., stearic acid, which will become uniformly dispersed within the aluminum base matrix powder during processing, and subsequently will decompose during vacuum hot degassing and/or hot consolidation, e.g., extrusion, forging, rolling, and form carbides and oxide particles dispersed within the matrix.
Although carbidiferous agents, said to be necessary for the mechanical alloying of aluminum base alloys, can become constituents in the final product (see, for example U.S. Pat. No. 4,627,959), prior art teachings suggest that the resulting Al.sub.4 C.sub.3 particles are not suitable for use at temperatures greater than 100.degree. C. Specifically, it has been taught that upon exposure to temperatures above 100.degree. C., age hardened structures and/or work hardened structures tend to soften. At higher temperatures the dispersion of Al.sub.4 C.sub.3 in the alloy is said to coarsen, thus lessening the contribution of carbide to the strength of the alloy. In consequence, aluminum base alloys of the prior art as produced by mechanical alloying are said to be generally unsuitable for use in the temperature range of 100.degree. C. to 500.degree. C. These aluminum carbides and oxides will provide further reinforcements in mechanical and physical properties at ambient and elevated temperatures. Prior processes in which aluminum based alloys and/or metal matrix composites are mechanically alloyed by means of solid state bonding are disclosed in U.S. Pat. Nos. 4,722,751, 4,594,222 and 3,591,362.
For the above reasons, in use of a carbidiferous processing aid, it has been proposed (see U.S. Pat. No. 4,624,705) that strong carbide formers such as titanium be added to produce in the final alloy carbides more thermally stable than Al.sub.4 C.sub.3 at temperatures in excess of 100.degree. C.